Spontaneous action potentials produced by Na and Cl channels in maturing Rana pipiens oocytes

The electrical excitability of maturing Rana pipiens oocytes was studied using intracellular recording and voltage-clamp techniques. Naturally ovulated oocytes, removed from the body cavity within a few hours after ovulation, possess voltage-sensitive Na and Cl channels that can produce action potentials (ap's). Young oocytes (sometime during metaphase I to first polar body stage) can generate trains of spontaneous action potentials: no chemical treatment or current injection is required. This is the first report of spontaneous repetitive firing in an egg cell membrane. As the oocyte matures, the duration of each ap increases because the outward Cl- current decreases. Middle-aged oocytes (about first polar body stage to metaphase II) have continuously positive membrane potentials (Vm's). Mature, activatable (metaphase II) oocytes have negative Vm's when impaled but can produce a long-lived ap when depolarizing current is injected. The ap's differ fundamentally from ap's in other excitable cells, including eggs: the Na+ current develops slowly and does not inactivate; most of the outward current is carried by Cl-, not by K+; the Cl channel is lost or is rendered insensitive to voltage as the oocyte matures.